Silicon-niobium alloys



United States Patent Office R 3,166,409 Patented Jan. .19, 196.5

This invention relates to niobium alloys. ly concerned with oxidationresistant alloys.

There are niobium alloys which retain their mechanical properties athigh temperatures, but they are not resistant to oxidation. The mostoxidation resistant strong niobium alloys are at least ten times moreresistant to oxidation than pure niobium, but their oxidation rates arestill about fifty times higher than could be tolerated inservice.

Those niobium alloys which have sufficiently low oxidation rates aregenerally based on brittle intermetallic compounds. However, most ofthese alloys oxidise more rapidly at intermediate temperatures than athigh temperatures, producing a low temperature oxidation rate peak. Thispeak may be shown by isothermal and slow thermal cyclic testing, both ofwhich conditions may be encountered in service.

One of the most oxidation resistant binary intermetallic compounds ofniobium in the disilicide Nbsi which is fairly resistant to hightemperature oxidation but suffers from an oxidation rate peak at1000-1050" C.

One object of the present invention is to improve the high temperatureoxidation resistance and to reduce the oxidation rate peak of niobiumdisilicide by ternary alloying additions.

Accordingly the present invention provides alloys based on theintermetallic compound niobium disilicide, in which part of the niobiumis replaced by at least one of the metals aluminum, chromium, iron ortitanium, these addition elements being present in such amounts as tomaintain the disilicide stoichiometry of the compound. Thus the alloysof the present invention are of the composition represented by theformula MSi Where M is niobium plus at least one of the additionelements listed, to a total of 33 /3 atomic percent. The silicon contentis always about 66 /3 atomic percent and preferably does not vary bymore than plus or minus two atomic percent.

We have found that such alloys possess high oxidation resistance both atelevated temperatures and at lower temperatures and are resistant tooxidation during cycles of heating and cooling, especially after a hightemperature oxide film has been formed.

We have found that small additions of the metals listed are effectiveand that the improvement in properties is maintained when over half ofthe niobium has been replaced. In particular we have found that up to23% atomic percent of the addition elements reduces the rate of'oxidation. Although some improvement in oxidation resistance is obtainedover the whole range there are certain limitations to the amounts ofaddition for optimum beneficial effect. Alloys containing aluminum inexcess of about have low melting point constituents which are exuded onthe surface at high temperatures. Alloys containing less than about 6%atomic percent titanium It is especial- Properties of certain alloysexemplifying the invention are listed in the accompanying table.

All the alloys exemplified are based on the intermetallic compoundniobium disilicide containing 33 /3 atomic percent niobium and 66%atomic percent silicon. The

. composition of the alloys is of the general form MSi deteriorate dueto severe oxidation and thickening of the where M represents niobiumplus one of the addition elements aluminum, chromium, iron, titanium,the total atomic percentage of M being 33 /3 .For comparison, resultsare also given for niobium disilicide without addition.

Test 1 was performed as follows:

This test indicatesthe formation of an initial high temperature oxidefilm on the surface. After the film has been formed the rate ofoxidation is reduced.

Test 2 was performed as follows:

'(a) Samples from Test 1 (i.e., with an adherent oxide film present).(12) Heated for 24 hours at 1200 C. (0) Results expressed as weight gainper unit surface area er hour (mg/sq. cm./hr.).

This test was designed to show the oxidation rate at a constanttemperature of 1200 C. after an initial oxide film has been formed.

Test 3 was performed as follows:

(a) Samples from Test 2.

(b) Heated at 1200 C..for 15 min.

(0) Cooled to 600 C. in 60 min.

(d) Heated from 600 C. to 1200? C. at 6 C./min. (e) Results expressed asin Test 1(f) This test shows the behaviour on heating and cooling and isthe most significant test, since it indicates the resistance tooxidation over a range of temperatures under conditions of heating andcooling typical of those encountered in service.

Test 4 was performed as follows:

(a) Samples from Test 3. (b) Heated for hrs. at 1100 C. (0) Resultsexpressed as in Test 2(a) This test shows the long-term oxidationcharacteristics including the effect of oxygen contamination, thepossibility of the oxide film spalling off if it becomes too thick, andthe presence of low melting point constituents.

It will be seen from the results that all the alloys listed are moreresistant to oxidation than niobium disilicide. The rate of oxidation ofniobium disilicide itself is less than that of niobium by a factor ofabout 50. The properties revealed by Test 3 are particularlysignificant.

Alloys containing more than 10 atomic percent of aluminium contain lowmelting point phases which were exuded as globules on the surface duringoxidation at 1100 C. and 1200 C. Alloys containing 3 /3 and 6 /3 atomicpercent chromium formed an oxide film which spalled on cooling to roomtemperature. Alloys containing up to 6% atomic percent of titaniumsuffered from spalling of the oxide scale.

The alloys of the present invention are brittle and diflicult tofabricate but they can be employed in the cast 4. A niobium-siliconalloy consisting essentially of form or used as oxidation resistantcoatings on other nio 64 /3 to 68% atomic percent silicon, 1% to 23 /3atomic bium alloys. a percent iron and the balance niobium.

Atomic percent Test 1 Test 2 Test 3 Test; 4 p

' V Condition Addition Addition Oxidation Oxidation Oxidation Oxidationof film element Nb i1% Si =i:2% element rate, mg./ rate, mg./ rate,ing./ rate, mg./ i :l:%% sq. cmJhr. sq. cmJhr. sq. cm./l1r. sq. cm./hr.

31% 66% 1% 0. 11 O. 019 N N VG Al 30 66% 3% 0.12 N N Nv VG 26% 66% 6% 0.11 0. 03 0. 03 0. 014 GL 31% 66% 1% 0. 10 0. 014 0. 06 0.015 GL 66% 3%0. 22 0.03 0. 10 SL Or 66% 6% 0. 12 N N 0. 03 SL 23% 66% 10 0.17 0.05 N0.003 VG 16% 66% 16% 0. 21 0. O 0. 09 0. 002 VG 10 66% 23% 0. 35 0. 0760. 04 0. 022 S 31% 66% 1% 0. N N -Q VG 30 66% 3% O. 48 N N 0. 001 GL 26%66%. 6% 0.29 N 0.06 0. 001 G Fe 23% 66% 0. 0. 03 0. 02 0. 006 GL 16% 66%16% 0. 17 0. 03 0. 02 N GL 66% 23% 0.30 0. 6 22 0. 022 0. 60* F 66% 33%N N .r F

26% 66% 6% 0. 11 p 0. 01 0. 07 0. 68 S1 23% 66% 10 0. 13 0. 01 0.02 0.603 GL Ti 16% 66% 16% 0. 18 0. 02 0. 09 N VG 10 66% 23% 0. 08 0. 034 N NF 66% 33% N N F N=Negligible oxidation, weightgain less than 0.1 mgJcm.during test. Condition of Film- VG=Very good adherent uniform film.SL=Film spelled in places due to localised oxidation. I GL=Good film,but some slight localised oxidation due to inhomogeneity. F= Glassy filmof silica or silicate. S1=Severe spelling on cooling to roomtemperature. S =Slight spalling I claim: 35 5. A niobium-silicon alloyconsisting essentially of 1. A niobium-silicon alloy consistingessentially of: 64 /3 to 68% atomic percent silicon, 6 /3 to 23 /3atomic 64% to 68 /3 atomic percent silicon, an addition agent sepercenttitanium and the balance niobium. lected from the group consisting of 1%to 10 atomic aluminum, 1 /3 to 23 /3 atomic percent chromium, 1 /3 toReferences C'ted b the Examin 23 /3, atomic percent iron and 6% to 23 /3atomlc percent 40 l y 7 er titanium, and the balance essentiallyniobium. UNITED STATES PATENTS 2. A niobium-silicon alloy consistingessentially of 2,169,193 8/39 Cornstock 75--134.3 64 /3 to 68 /3 atomicpercent silicon, 1% to 10 atomic 2,939,785 6/66 Weatherly et a1. 75-474percent aluminum and the balance niobium.

3. A niobium-silicon alloy consisting essentially of DAVID RECK PnmaryExaminer 64 /3 to 68% atomic percent silicon, 1 /3 to 23% atomic WINSTONPQUGLAS, Examiner, percent chromium and the balance niobium. i l

1. A NIOBIUM-SILICON ALLOY CONSISTING ESSENTIALLY OF: 64 2/3 TO 68 2/3ATOMIC PERCENT SILICON, AN ADDITION AGENT SELECTED FROM THE GROUPCONSISTING 1 2/3 TO 10 ATOMIC ALUMINUM, 1 1/3 TO 23 1/3 ATOMIC PERCENTCHROMIUM, 1 1/3 TO 23 1/3 ATOMIC PERCENT IRON AND 6 2/3 TO 23 1/3 ATOMICPERCENT TITANIUM, AND THE BALANCE ESSENTIALLY NIOBIUM.